1. The Field of the Invention
This invention relates to the field of printed feature manufacturing, such as integrated circuit manufacturing. In particular, this invention relates to automatically identifying evaluation points where errors are computed and analyzed to achieve improved agreement between a design layout and an actual printed feature.
2. Description of Related Art
To fabricate an integrated circuit (IC), engineers first use a logical electronic design automation (EDA) tool, also called a functional EDA tool, to create a schematic design, such as a schematic circuit design consisting of symbols representing individual devices coupled together to perform a certain function or set of functions. Such tools are available from CADENCE DESIGN SYSTEMS and from SYNOPSYS. The schematic design must be translated into a representation of the actual physical arrangement of materials upon completion, called a design layout. The design layout uses a physical EDA tool, such as those available from CADENCE and AVANT!. If materials must be arranged in multiple layers, as is typical for an IC, the design layout includes several design layers.
After the arrangement of materials by layer is designed, a fabrication process is used to actually form material on each layer. That process includes a photo-lithographic process using a mask having opaque and transparent regions that causes light to fall on photosensitive material in a desired pattern. After light is shined through the mask onto the photosensitive material, the light-sensitive material is subjected to a developing process to remove those portions exposed to light (or, alternatively, remove those portions not exposed to light). Etching, deposition, diffusion, or some other material altering process is then performed on the patterned layer until a particular material is formed with the desired pattern in the particular layer. The result of the process is some arrangement of material in each of one or more layers, here called printed features layers.
Because of the characteristics of light in photolithographic equipment, and because of the properties of the material altering processes employed, the pattern of transparent and opaque areas on the mask is not the same as the pattern of materials on the printed layer. A mask design process is used, therefore, after the physical EDA process and before the fabrication process, to generate one or more mask layouts that differ from the design layers. When formed into one or more masks and used in a set of photolithographic processes and material altering processes, these mask layouts produce a printed features layer as close as possible to the design layer.
The particular size of a feature that a design calls for is the feature's critical dimension. The resolution for the fabrication process corresponds to the minimum sized feature that the photolithographic process and the material processes can repeatably form on a substrate, such as a silicon wafer. As the critical dimensions of the features on the design layers become smaller and approach the resolution of the fabrication process, the consistency between the mask and the printed features layer is significantly reduced. Specifically, it is observed that differences in the pattern of printed features from the mask depend upon the size and shape of the features on the mask and the proximity of the features to one another on the mask. Such differences are called proximity effects.
Some causes of proximity effects are optical proximity effects, such as diffraction of light through the apertures of the optical systems and the patterns of circuits that resemble optical gratings. Optical proximity effects also include underexposure of concave corners (inside corners with interior angles greater than 180 degrees) and overexposure of convex corners (outside corners with interior angles less than 180 degrees), where the polygon represents opaque regions, and different exposures of small features compared to large features projected from the same mask. Other causes of proximity effects are non-optical proximity effects, such as sensitivity of feature size and shape to angle of attack from etching plasmas or deposition by sputtering during the material altering processes, which cause features to have shapes and sizes that have decayed from or accumulated onto their designed shapes and sizes.
In attempts to compensate for proximity effects, the mask layouts can be modified. To illustrate, FIG. 1A shows mask items 170, such as windows or opaque areas on a mask, represented by edges 171 on one or more polygons, and some printed features 180 with exaggerated proximity effects. FIG. 1A does not represent an actual example, but is provided simply to illustrate the concepts of proximity effects and mask modifications to attempt to correct for proximity effects. To make apparent the illustrated proximity effects, the projection of the original polygons 171 is shown as a fine line around the printed features 180. Note that the printed features 180 includes a spurious connection feature 182, an edge 185 entirely displaced inside the corresponding edge of the original outline 171, and an end line 183 completely inside the outline of the original items 171.
FIG. 1B illustrates various ways mask items 190 are modified to correct for such effects. FIG. 1B is not a particular example of a particular set of corrections that actually mitigate the proximity effects illustrated in FIG. 1A. The corrections available include hammerheads 192 (i.e. hammerheads 192a and 192b) added to ends of items to compensate for overexposure of the entire end line of a feature. Also shown are biases 196 (i.e. biases 196a and 196b) applied along portions of a straight edge of a feature. A negative bias like 196 represents a portion of an opaque area made transparent (or a portion of a window made opaque). In this case the negative bias reduces the size of the item on a mask to avoid generating the spurious feature 182 of FIG. 1A. Also shown are assist features 194 (i.e. assist features 194a and 194b), which are separate items smaller than the resolution of the photolithographic process and thus too small to be formed in a photoresist layer, but which are sufficiently large to effect diffraction patterns that influence larger nearby features. The assist features 194 are intended to move the edge 185 of the printed features 180 in FIG. 1A toward the outline 171 of the original mask items 170. Also shown is a sub-resolution serif 193 of extra opaque material to compensate for overexposure at convex corners of opaque areas, and an anti-serif 197 indicating where opaque material, if any, is removed to compensate for underexposure at concave corners of opaque polygons. These corrections are listed to illustrate the concepts of correcting a mask to compensate for proximity effects. The illustrated corrections do not necessarily correct the depicted features.
In rule-based proximity corrections, corrections such as the serifs and biases of a predetermined size are automatically placed at corners and edges of fabrication layout shapes like the mask shapes 170. Other rules may include adding assist shapes to a mask near desired features from the design layout and adding hammerheads to short end lines of the desired features. Experience of engineers accumulated through trial and error can be expressed as rules, and applied during the fabrication design process. For example, a rule may be expressed as follows: if a feature is isolated, then widen the opaque region in the mask by a particular amount to compensate for expected overexposure, so that the feature prints properly.
The experience captured in rule-based corrections is garnered by going through the fabrication process repeatedly with different mask layouts, making adjustments and observing the results. However, this process consumes time and manufacturing capacity. Even if such experimental, rule-forming runs are made, the resulting rules often do not give satisfactory results as the features become more complicated, or become smaller, or interact in more confined areas, or involve any combination of these effects.
Because rule based corrections are often imprecise and time consuming to revise and test, a proximity effects model is often developed and used to predict the effects of a change in the mask layout with more precision and with fewer off line fabrication runs.
A proximity effects model is typically built for a particular suite of equipment and equipment settings assembled to perform the fabrication process. The model is often built by performing the fabrication process one or a few times with test patterns on one or more mask layouts, observing the actual features printed (for example with a scanning electron micrograph), and fitting a set of equations or matrix transformations that most nearly reproduce the locations of edges of features actually printed as output when the test pattern is provided as input. The output of the proximity effects model is often expressed as an optical intensity. Other proximity effects models provide a calibrated output that includes a variable threshold as well as an optical intensity. Some proximity effects models provide a calibrated output which indicates a printed edge location relative to a particular optical intensity as a spatial deviation. Thus some model outputs can take on values that vary practically continuously between some minimum value and some maximum value.
Once a proximity effects model is produced, it can be used in the fabrication design process. For example, the proximity effects model is run with a proposed mask layout to produce a predicted printed features layer. The predicted printed features layer is compared to the design layer and the differences are analyzed. Based on the differences, modifications to the proposed mask layout are made, such as by adding or removing serifs 193, or adding or removing anti-serifs 197. After making these corrections to the proposed mask layout, a final mask layout is generated that is used in the fabrication process.
A proximity effects model typically produces predictions that lead to corrections that are more accurate than rule based corrections. However, a proximity effects model consumes computational resources by an amount that is related to the number of points of interest where amplitudes are computed. For typical mask layouts, the number of points where the model could be run is large, and the computation time is prohibitive. Therefore it is typical to run the proximity effects model at selected evaluation points located on the edges of features, to determine the correction needed, if any, at each evaluation point, and then to apply that correction to all the points on a segment of the edge in the vicinity of the evaluation point.
If too many evaluation points are selected, the model runs too slowly and the correction procedure takes too long. If too few evaluation points are selected, the model may not detect critical locations where corrections are necessary. Another undesirable effect of too few evaluation points is that segments can become too large, so that even if a correction is accurate for the evaluation point, the correction is applied to too much of the edge. This causes undesirable changes on the edge away from the evaluation point.
What are needed are new ways to select evaluation points and define their vicinity for the mask layouts, so that a reasonable number of comparisons can be made and so that effective modifications to the mask layouts can be suggested.